1. Field of the Invention
The present invention relates to an apparatus for measuring an insulation resistance characteristic of a capacitive electronic part such as a capacitor.
2. Description of the Related Art
A measuring apparatus shown in FIG. 1 has been used to measure an insulation resistance characteristic of a capacitive electronic part such as a capacitor in the past. Specifically, a direct current (dc) measurement power supply 1 has one terminal thereof grounded and the other terminal thereof connected to one terminal of a measured capacitor 3 via a current-limiting resistor 2. One terminal of a voltmeter 4 is connected between the current-limiting resistor 2 and measured capacitor 3. The other terminal of the measured capacitor 3 is connected to an ammeter 5, and a leakage current flowing through the measured capacitor 3 is measured using the ammeter 5.
A measurement voltage E applied to the measured capacitor 3 is measured using the voltmeter 4, and a current is flowing through the measured capacitor 3 is measured using the ammeter 5. An insulation resistor R characteristic of the measured capacitor 3 can be calculated as follows:
R=E/I
In the foregoing measuring apparatus, a measured value of the insulation resistance R may contain an error because of a noise caused by the measurement power supply 1, a hum caused by a power supply or the like, and a noise caused by the measured capacitor 3 itself. This poses a problem.
Reasons why the error occurs will be described with reference to FIG. 2.
Assume that the capacitance of the measured capacitor 3 is C, an insulation resistance is R, a dc voltage generated by the measurement power supply 1 is E, and a voltage (alternating voltage component) derived from a noise caused by the measurement power supply 1 or a hum caused by a mains power supply is e.
Except for the noise component e, I=Ir (leakage current) would be established and the insulation resistance R characteristic of the measured capacitor 3 could be calculated according to R=E/I. However, in reality, the current I does not equal the leakage current Ir determined with the insulation resistance but contains a noise component Ic flowing through the capacitor. Namely, I=Ir+Ic is established. This results in an error of a measured value.
For example, assume that an insulation resistance, characteristic of a capacitor, exhibiting an insulation resistance of 50 Mxcexa9 and offering a capacitance of 10 xcexcF, is measured using a voltage of 50 V, and an output of a power supply contains a noise of 10 mVrms. In this case, the leakage current Ir and noise component Ic are calculated as follows:
Ir=50 V/50 Mxcexa9=1 xcexcA
Ic=10 mVrms/(xc2xdxcfx80xc3x9760xc3x9710 xcexcF)=approx. 38 xcexcArms
The current of 1 xcexcA that should be measured is buried under the noise current 38 xcexcA that is 30 or more times larger than the current of 1 xcexcA. This does not allow precise measurement. If the values of the current I detected for a long period of time are integrated, an average of the values of the noise component Ic of the current I approaches 0. Measurement now becomes possible. However, it takes too much time to complete the measurement. This poses a problem.
When a resistor Rs is, as shown in FIG. 3, inserted in the middle of a path leading from the measured capacitor 3 to the ammeter 5, the noise component Ic can be reduced. For example, assuming that Rs equals 50 kxcexa9, Ir and Ic are calculated as follows:
Ir=50 V/(50 Mxcexa9+50 kxcexa9)=approx. 1 xcexcA
Ic=10 mVrms/(50 kxcexa9+xc2xdxcfx80xc3x9760xc3x9710 xcexcF)=approx. 0.2 xcexcArms
The noise component is as small as 0.2 xcexcA relative to the current to be measured, 1 xcexcA. This enables precise measurement. However, since the resistor offering a resistance that is as large as 50 kxcexa9 is used, a charging current used to charge the capacitor with the capacitance C flows for a time that is several times longer than a time, constant RC=50 kxcexa9xc3x9710 xcexcF=500 ms. Measurement cannot be started until the charging current ceases. It therefore takes too much time to complete the measurement. This poses a problem.
An object of the present invention is to provide an insulation resistance measuring apparatus capable of highly precisely measuring an insulation resistance characteristic of a capacitive electronic part in a short period of time while being unaffected by various kinds of noises.
For accomplishing the above object, according to the first embodiment of the present invention, there is provided an insulation resistance measuring apparatus that applies a predetermined measurement voltage to a capacitive electronic part, and measures a current flowing through the electronic part so as to calculate an insulation resistance characteristic of the electronic part. Herein, a noise clipper circuit is connected on a path leading from a measurement power supply through the capacitive electronic part to a current detector. The noise clipper circuit is realized with a circuit having a resistor Ra and a switching means RE1 connected in parallel with each other. The switching means RE1 is controlled to remain closed in an early stage of charging the capacitive electronic part. When charging the capacitive electronic part has progressed sufficiently, the switching means RE1 is controlled to open.
For achieving precise measurement in a short period of time, while a large current is flowing through the noise clipper circuit, or in other words, while a charging current used to charge a capacitor is flowing, a resistance Ra should be reduced. When the current flowing through the noise clipper circuit gets smaller, or in other words, when the charging current used to charge the capacitor ceases and only a current determined with the insulation resistance R flows, the resistance Ra should be increased. According to the first embodiment of the present invention, a noise clipper circuit consists of a resistor Ra and a switching means RE1 which are connected in parallel with each other. In an early stage of charging a capacitive electronic part, the switching means RE1 is closed. When charging the capacitive electronic part has progressed sufficiently, the switching means RE1 is opened.
In the early stage of charging the capacitive electronic part, since the switching means RE1 is closed, the noise clipper circuit offers almost no resistance. In other words the resistance offered by the noise clipper circuit is so small that the capacitive electronic part can be charged smoothly. Thereafter, when charging the capacitive electronic part has been nearly completed, the switching means RE1 is opened. This causes the resistance offered by the noise clipper circuit to increase. A noise component of a charging current is cut off. This results in precise measurement.
For measuring a final insulation resistance characteristic of a capacitive electronic part, the noise clipper circuit composed of the resistor Ra and switching means RE1 alone will suffice. For detecting a temporal variation of the charging current for charging the capacitive electronic part, when the charging current gets smaller with the switching means RE1 closed, the variation of the charging current may not be measured correctly because of the adverse effect of a noise current. However, when charging the capacitive electronic part is performed insufficiently, if the switching means RE1 is opened, charging the capacitive electronic part is retarded because of the resistor Ra.
According to the second embodiment of the present invention, preferably, a noise, clipper circuit is realized with a parallel circuit having a first resistor Ra, a first switching means RE1, and a series circuit connected in parallel with one another. The series circuit consists of a second resistor Rb and a second switching means RE2. In this case, preferably, a resistor offering a resistance smaller than that offered by the first resistor Ra is used as the second resistor Rb.
In an early charging stage, both the switching means RE1 and RE2 are closed. When charging has progressed to some extent and a charging current has been reduced, the switching means RE1 is opened and the switching means RE2 is closed. A resistance offered by the noise clipper circuit corresponds to the sum of the resistances of the resistors Ra and Rb connected in parallel with each other. A noise current is suppressed to be negligible relative to a current detected in this stage. Moreover, since charging the capacitive electron part has progressed, considerably, if charging is somewhat interrupted, no critical situation will take place. However, when charging the capacitive electronic part has further progressed, if the charging current is reduced, the noise current gets relatively larger and becomes outstanding. At this time, when the second switching means RE2 is opened, the resistance offered by the noise clipper circuit corresponds to the resistance of the resistor Ra. The noise current is therefore very small. A leakage current can be measured precisely. Incidentally, the switching means RE2 need not always be closed in the early charging state. The switching means RE1 alone should be closed in the early charging state.
As for the series circuit composed of the second resistor Rb and second switching means RE2 in accordance with the second embodiment of the present invention, any required number of series circuits may be connected in parallel with one another. In this case, while the adverse effect of a noise current is avoided within a wide range of current values, a variation of a charging current can be measured.
A noise clipper circuit in accordance with the present invention may be connected between a measurement power supply and a capacitive electronic part. Preferably, the noise clipper circuit is connected between the capacitive electronic part and a current detector.
Noises with relatively low frequencies fall into those noise voltages which are thought to occur in series with a power supply and those noise voltages which are thought to occur in parallel therewith. The latter noises often occur near a measurement terminal that is brought into contact with the capacitive electronic part. The noises cause a noise current to flow from a point in a noise clipper circuit, at which the noises occur, towards a circuit element offering a lower impedance. When the noise clipper circuit is connected near the power supply rather than the capacitive electronic part, the noise current largely flows into a current detector. This causes an error. Consequently, the noise clipper circuit is connected near the current detector rather than the capacitive electronic part. This causes a noise current stemming from the latter noises to almost entirely flow into the power supply, so that measurement of a current can be achieved while being unaffected by the noise current. For the same reason, preferably, a current-limiting resistor is connected near the current detector rather than the capacitive electronic part.
However, even if the noise clipper circuit is connected near the power supply rather than the capacitive electronic part, the noise clipper circuit works effectively on noises occurring in series with the power supply.
The switching means RE1 and RE2 are closed in the early charging stage. When charging has progressed to some extent, the switching means are opened. For determining the timing of opening, a current value measured by the current detector may be compared with a predetermined threshold value. When the current value measured by the current detector falls below the predetermined threshold value, the switching means may be opened. Alternatively, a time having elapsed since the start of charging may be measured, and the switching means may be opened in a predetermined time after the start of charging.
According to the third embodiment of the present invention, there is provided an insulation resistance measuring apparatus that applies a predetermined measurement voltage to a capacitive electronic part, and measures a current flowing through the electronic part so as to calculate an insulation resistance characteristic of the electronic part. A noise clipper circuit is connected on a path leading from a measurement power supply through the capacitive electronic part to a current detector. The noise clipper circuit is realized with a circuit having a resistor Ra and a diode Da connected in parallel with each other. The anode of the diode Da is connected on the side of the positive voltage terminal of the measurement power supply.
For achieving precise measurement for a short period of time while a large current is, flowing through a noise clipper circuit, that is, while a charging current used to charge a capacitor is flowing, a resistance is reduced. When the current flowing through the noise clipper circuit decreases, that is, when the charging current used to charge the capacitor ceases and only a current determined with an insulation resistance R is present, the resistance is increased. According to the third embodiment of the present invention, the resistor Ra and diode Da connected in parallel with each other are included to constitute a noise clipper circuit, and the anode of the diode Da is directed towards the measurement power supply.
Diodes (semiconductor devices having a pn junction) have the following properties: when a forward voltage is applied, a current hardly flows until the voltage, equals a predetermined voltage (this means that a resistance is large); and when the voltage exceeds the predetermined voltage, a large current flows (this means that the resistance is small). In an early stage of charging a capacitive electronic part or in a stage in which a current is large, the current almost entirely flows through the diode Da. The electronic part can therefore be charged smoothly. In contrast, when charging the capacitive electronic part has been nearly completed, the current flowing through the electronic part becomes very small. No current therefore flows through the diode Da. A current flows exclusively through the resistor Ra. In other words, since the noise clipper circuit offers a large resistance, a noise component of a charging current is cut off. This enables precise measurement.
According the fourth embodiment of the present invention, a noise clipper circuit is a circuit having a resistor Ra, a first diode Da1, and a second diode Da2 connected in parallel with one another. The, first diode Da1 and second diode Da2 share the same properties and are connected in mutually opposite directions.
Specifically, when a noise clipper circuit is realized with a parallel circuit composed only of a resistor and a diode, if a noise voltage is high, a small noise current flows through the diode in the forward direction. However, a current barely flows in the backward direction because of the diode. A circuit for filtering a measured current may be adopted. Otherwise, a current may be measured continuously and the results of measurement may be averaged or processed digitally. In this case, since a noise current flows in the forward direction alone, a measured value of a leakage current becomes larger than a true value.
In reality, when measurement is performed, charging of a capacitor, that is an electronic part, has been nearly completed. A dc voltage applied to a noise clipper circuit is usually 0 V, and a noise voltage e assumes any value near 0 V. Therefore, diodes may be connected in parallel with each other in positive and negative directions. In this case, a noise current flowing in the forward direction and a noise current flowing in the backward direction assume almost the same value. When a measured current is filtered or measured current values are processed digitally, a nearly true leakage current can be determined. Thus, a true insulation resistance can be measured.
According to the fifth embodiment of the present invention, a noise clipper circuit is realized with a circuit having a first resistor Ra, a series circuit composed of a second resistor Rb and a first diode Db, and a second diode Da connected in parallel with one another. The anodes of the first and second diodes Db and Da are connected on the side of the positive voltage terminal of a measurement power supply. A forward voltage drop occurring in the first diode Db is smaller than that occurring in the second diode Da. The resistance of the first resistor Ra is larger than that of the second resistor Rb.
According to the third embodiment of the present invention, when charging a capacitive electronic part has been nearly completed and a current flowing through the electronic part has become very small, no current flows through the diode. The resistance offered by the noise clipper circuit becomes equal to that offered by the resistor Ra. However, in this stage, the electronic part has been charged to develop a voltage which is lowered by a forward voltage than the measurement voltage. The electronic part must therefore be further charged by an amount of charge proportional to the forward voltage through the resistor Ra. This would become an obstacle to more speedy measurement. For shorting the measurement time, a diode causing only a small forward voltage drop should be used. However, a noise voltage is likely to be applied to such a diode causing only a small forward voltage drop.
According to the fifth embodiment of the present invention, the series circuit, composed of the diode Db causing a small forward voltage drop and the resistor Rb, is connected in parallel with the noise clipper circuit described in the third embodiment. A little noise current flowing through the diode Db causing a small forward voltage drop is cut off using the resistor Rb. Consequently, the electronic part is charged more smoothly through the diode Da in the early charging stage. Thereafter, the electronic part is charged through the series circuit composed of the diode Db and resistor Rb. Finally, the resistor Ra is used to perform charging and measurement.
This results in a noise clipper circuit having the optimal properties of not hindering charging of an electronic part during charging and not conducting a noise current during measurement.
According to the sixth embodiment of the present invention, an oppositely-directed zener diode ZDa is substituted for the second diode Da employed according to the fifth aspect. As already known, when a reverse voltage is applied to the zener diode ZDa, a current barely flows through the zener diode ZDa. When the voltage exceeds a predetermined value (breakdown voltage), a large reverse current flows abruptly. Due to the properties of the zener diode, the use of the zener diode ZDa provides the same operation and advantage as those exerted by the noise clipper circuit described in the fifth embodiment.
According to the seventh embodiment of the present invention, a noise clipper circuit consists of a first resistor Ra, a series circuit composed of a second resistor Rb and an opposite parallel circuit having a diode Db1, and a diode Db2 connected in parallel with each other in mutually opposite directions, and an opposite parallel circuit having a diode Da1 and a diode Da2 connected in parallel with each other in mutually opposite directions. The first resistor Ra, series circuit, and opposite parallel circuit are connected in parallel with one another. The diode Da1, diode Da2, diode Db1, and diode Db2 share the same properties. A forward voltage drop occurring in the diode Db1 or Db2 is smaller than that occurring in the diode Da1 or Da2. The resistance of the first resistor Ra is larger than that of the second resistor Rb.
This results in a noise clipper circuit capable of exerting the operations and advantages exerted by the noise clipper circuits described in the fourth and fifth, embodiments.
According to the eighth embodiment of the present invention, an opposite series circuit having a zener diode ZDa1 and a zener diode ZDa2 connected in series with each other in mutually opposite directions is substituted for the opposite parallel circuit composed of the diode Da1 and diode Da2 described in the seventh embodiment. Consequently, the same operation and advantage as those exerted by the noise clipper circuit described in the seventh embodiment can be exerted.
A noise clipper circuit in accordance with the present invention, may be connected between a measurement power supply and a capacitive electronic part. Preferably, the noise clipper circuit is connected between the capacitive electronic part and a current detector.
Noises with relatively low frequencies fall into those noise voltages which are thought to occur in series with a measurement power supply and those noise voltages which are thought to occur in parallel with the measurement power, supply. The latter noises usually occur near a measurement terminal that is brought in contact with an electronic part. The noises cause a large noise current to flow from a point in a noise clipper circuit, at which the noises occur, to a circuit element offering a lower impedance. When the noise clipper circuit is connected near the power supply rather than the capacitive electronic part, a large noise current flows into a current detector. This causes an error. Therefore, the noise clipper circuit is connected near the current detector rather than the electronic part. This causes the noise current stemming from the latter noises to almost entirely flow into the power supply. Measurement of a current will therefore be unaffected by the noise current.
For the same reason, preferably, a current-limiting resistor is connected near the current detector rather than the electronic part.
However, even when the noise clipper circuit is installed near the power supply rather than the electronic part, the noise clipper circuit will prove effective for noises that occur in series with the power supply.